Irregular Variables

Table 5.9 includes the brightest of the irregular variables, whose sudden brightenings, or decreases in brightness, are unpredictable. They include the R CrB (for R Coronae Borealis) and S Dor (for S Doradus) classes. R CrB stars are known for sudden dips in brightness followed by slower recovery to normal levels; there is evidence for ejected clouds of carbon during such episodes. S Dor stars are super-giants that undergo shell ejection. The prototype for this class is S Doradus, among the most intrinsically luminous stars (Hoffmeister et al. 1985, p. 176); it is located in the Large Magellanic Cloud. Related to S Dor stars are stars that, although much closer to the main sequence in the Herzsprung-Russell diagram, also undergo shell ejection episodes. One of the better known stars of this subclass is the "shell star" P Cygni, discovered by W. Blaeuw in 1600. P Cygni underwent a visual brightness change of three magnitudes between 1597 and 1602. These variable stars are very luminous and blue, with large light amplitude and long time scales of variation. Spectra show evidence of past ejections of massive shells of gas. As is sometimes the case within classes of variable stars, the amplitude, time scale of variation, luminosity class, and spectral type of S Dor variables differ from object to object.

Another type of star shedding its atmosphere is the very hot Wolf-Rayet (WR) class; this kind of object too is observed to undergo irregular light variations.

The star g Cass has an amplitude of 1.4 magnitudes (Hoffmeister et al. 1985, p. 178); its spectrum shows evidence of a rapidly rotating disk or shell. One of the Pleiades stars, BU Tau, better known as Pleione, undergoes shell ejection with a small light variation. Other stars in the Pleiades may also be shell stars, and some of the speculation about the "lost Pleiad" has involved this type of variability.

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